Adsorption is a process whereby one or more components (an adsorbate) in a fluid phase are transfer to, and accumulate on, the surface of a solid adsorbent. Desorption is the reverse process of adsorption and involves the transfer of an adsorbate from a solid adsorbent to a fluid phase.
The process of adsorption is exothermic while the reverse desorption process is endothermic. As adsorption and desorption are heat exchange processes, adsorption and desorption can be utilised in heating and cooling applications.
In recent years there has been growing interest in the use of “adsorption chillers” that utilise water as a refrigerant. A major advantage of these adsorption chillers is that they can chill water in an adsorption mode and then utilise “waste heat” to regenerate the adsorbent in a desorption mode. Known adsorption chillers are capable of utilising heat as low as 70° C. to produce cooled water in the range of 5° C. to 15° C.
A schematic diagram of a known adsorption chiller is shown in FIG. 1. This adsorption chiller comprises two reactor chambers 10 and 20 respectively having two heat exchangers 12 and 22 that are disposed within a solid adsorbent bed. The two reactor chambers 10 and 20 are housed within a vacuum chamber and are in fluid communication with a condenser 30 that has a plurality of copper fin tube and shell tubes 30a that are in fluid communication with a cooling water source 32. The reactor chambers 10 and 20 are also in fluid communication with an evaporator 40 that has a plurality of copper fin tube and shell tubes 40a that are thermally coupled to a falling film type heat exchanger 42. The heat exchanger 42 accommodates the chilled water stream.
The evaporator 40 and condenser 30 are capable of being in fluid communication with the respective chambers 10 and 20 by large diameter vapor valves (14a,14b,14c,14d). The large diameter vapor valves (14a,14b,14c,14d) are provided to reduce the pressure drop between the reactor chambers 10 and 20 operating in vacuum and the condenser 30 and the evaporator 40.
The adsorption chiller of FIG. 1 is shown operating with the reactor chamber 10 in adsorption mode and the reactor chamber 20 in desorption mode. The operation of the adsorption chiller is as follows. The reactor chamber 10 charged with the solid adsorbent adsorbs water vapor that has been evaporated from liquid water brought into the evaporator 40. The evaporated water from the evaporator 42 passes through open valve 14a to the reactor chamber 10 (valve 14D is closed).
Evaporation of liquid water from the evaporator 40 is driven by the adsorption process, the heat transferred by the latent heat of evaporation causes the fluid in heat exchanger 42 to cool down. The cooled water exchanges heat with water passing through heat exchanger 42 so that the outlet water of heat exchanger 42 becomes “chilled water”. The chilled water from heat exchanger 42 may be used as a refrigerant for cooling applications.
In FIG. 1, the reactor chamber 20 is charged with an adsorbent material that has been charged or saturated with water vapor. Hot water from a heat source passes through heat exchanger 22 causing the reactor 10 to operate in the desorption mode whereby water vapor is driven off and passes through open valve 14b (valve 14C is closed) to be condensed in the condenser 30 as cooling water passes through heat exchanger 32.
At the end of the adsorption cycle in reactor 10 and the desorption cycle in reactor 20, the valves 14a and 14b are closed and valves (14C,14D) are open to reverse the process so that reactor chamber 10 operates in the desorption mode and reactor chamber 20 operates in the adsorption mode. Because the adsorption chiller is capable of operating in an adsorption mode and a desorption mode, the process is known as “regenerative adsorption”.
A disadvantage with known adsorption chillers is that the vapor valves (i.e. valves 14a,14b) are expensive and increase the cost of the adsorption chilling system. Furthermore, the use of copper shell and tubes within the condenser 30 and evaporator 40 for heat exchanges are expensive and add to the overall cost of the regenerative adsorption system.
A further disadvantage with known adsorption chillers is that the heat transfer coefficients of the heat exchangers is at most 3,500 W/m2K. Accordingly, large sized heat exchangers are required relative to the chilling duty of the adsorption chillers.
There is a need to provide a regenerative adsorption system that overcomes or at least ameliorates one or more of the disadvantages described above.
There is a need to provide a regenerative adsorption system that has a high heat transfer coefficient relative to that of an adsorption chillers as described above.
There is a need to provide a regenerative adsorption system that does not need to utilise vapor valves between the reactor chamber and the associated evaporator or condenser.